A hollow sounding plasterboard lining to an exterior wall should not be entered as ‘Internal Insulation’ unless you have documentary evidence to support your choice. RdSAP 2009 v 9.91 will require such a wall to be entered as ‘Dry lining’, but not ‘Internal Insulation’. Care should be taken to distinguish between Dry Lining and Timber Frame. Dry Lining will add thickness to the existing wall. A Timber Frame wall will have an outer skin, perhaps brick, and a timber/plasterboard inner skin separated by an insulation layer. This construction is likely to be thinner than a traditional wall with dry lining retro fitted.